1. Field of the Invention
The present invention relates in general to a driver circuit for a piezoelectric actuator, and a dot-matrix print head and a dot-matrix printer utilizing a piezoelectric actuator, and more particularly to a technique for controlling displacement of the piezoelectric actuator. The invention is also concerned with a dot-matrix printer using a print wire activated by a piezoelectric or other actuator, and more particularly with a technique for preventing unintended formation of dots due to a rebounding motion of the print wire which may be caused by a return movement of the wire from the advanced printing position back to the fully retracted position.
2. Discussion of the Related Art
A piezoelectric element oscillates at a given frequency when alternately charged and discharged together with a coil. In some applications of the piezoelectric element, it is required that the piezoelectric element be held in the same displaced state for more than a predetermined time duration. For example, this requirement should be satisfied when the piezoelectric element is used as an actuator for operating a print wire of a print head for a dot-matrix impact printer.
In view of the above requirement, the assignee of the present invention developed a driver circuit for a piezoelectric element, which is adapted to hold the piezoelectric element in a predetermined operating position for a sufficiently long time, without deteriorating the operating response. This driver circuit is disclosed in Japanese Patent Application No. 63-114397. An example of the driver circuit as disclosed in this document is illustrated in FIG. 16, in which a DC power source E, a first transistor TR1, a coil L and a piezoelectric element P are connected in series to each other. The negative terminals of the DC power source E and the piezoelectric element P are both grounded. The transistor TR1 allows an electric current to flow therethrough in the direction from the positive terminal of the power source E toward the positive terminal of the piezoelectric element P. This direction of the current flow will be referred to as "forward direction" of the circuit when appropriate. The positive terminal of the piezoelectric element P is also ground through a resistor R and a second transistor TR2. The second transistor TR2 allows an electric current to flow therethrough in the direction from the resistor R toward the ground.
The first and second transistors TR1, TR2 are turned on and off by a transistor controller TC. These transistors TR1, TR2 are normally placed in the off state. When the controller TC receives a drive command to drive the piezoelectric element P, the controller TC turns on the first transistor TR1. As a result, an electric energy is transferred from the DC power source E to the piezoelectric element P via the first transistor TR1 and the coil L, whereby the piezoelectric element is charged while oscillating with the coil L. Consequently, a voltage V.sub.P of the piezoelectric element P begins to rise from zero, as indicated in solid line in the graph of FIG. 17. The controller TC turns off the first transistor TR1 or returns the transistor TR1 to its original off state, when a predetermined time T.sub.C has passed after the beginning of charging of the element P. This time T.sub.C is equal to or longer than a half of a nominal activation cycle time of the element P which corresponds to the oscillation frequency determined by a product of the equivalent capacitance of the element P and the reactance of the coil L. Accordingly, the voltage V.sub.P of the piezoelectric element P reaches a peak level which is about two times as high as a line voltage V.sub.E of the power source E. Namely, the charging of the element P is completed when the voltage V.sub.P reaches the peak level indicated above. Even if the frist transistor TR1 remains on after the piezoelectric element P is completely charged, the discharging of the element P is prevented and the voltage V.sub.P is held constant, because the transistor TR1 does not allow an electric current to flow in the direction opposite to the forward direction, i.e., inhibits a flow of the current in the reverse direction from the element P toward the power source E. The time TC indicated in FIG. 17 is referred to as "charging time" during which the first transistor TR1 is held in the on state to charge the piezoelectric element P. After the charging time T.sub.C expires, the first transistor TR1 placed in the off state inhibits both the charging and the discharging of the element P, whereby the voltage V.sub.P can be maintained at the same level (peak level indicated above).
The transistor controller TC turns on the second transistor TR2 when a predetermined time has passed after the drive command is received. As a result, the electric energies stored in the piezoelectric element P and in the coil L are released and consumed by the resistor R. The controller TC returns the second transistor TR2 to its original off state at a point of time when the energies should have been completely consumed by the resistor R. A discharging time during which the second transistor TR2 is held on is indicated at T.sub.D in the graph of FIG. 17.
As is apparent from the above description, the driver circuit developed by the assigne:e of the present invention is capable of maintaining the maximum amount of displacement of the piezoelectric element P for a period from the moment of completion of the charging to the moment of commencement of the discharging, thereby enabling the piezoelectric element P to maintain a constant drive force so that the drive force acts on a subject to be driven, for a predetermined time equal to the period for which the element is held in its fully displaced state.
However, a continuing study of the present applicants revealed a following probler: with the driver circuit discussed above. Namely, the voltage V.sub.P of the piezoelectric element P is lowered as indicated in broken line in FIG. 17, due to consumption of the electric energy by the element P during working of the element P on the subject, since the element P in its fully displaced state is inhibited from being charged any longer. As a consequence, the amount of displacement of the piezoelectric element P is reduced and the resulting drive force is accordingly reduced.
Further, if the piezoelectric element P cannot be completely discharged in a given activation cycle for some reason or other, an electric energy may remain in the element P at the start of charging in the next activation cycle. In this case, the amplitude of oscillation of the coil L and the element P is reduced due to the remaining energy, and the voltage V.sub.P of the element P cannot reach the nominal peak level which is normally obtained when the element P is charged without an energy left therein. Thus, the piezoelectric element P does not produce the nominal drive force. Where the piezoelectric element P is used for activating a print head, for example, the piezoelectric element is alternately turned on and off in a repeated fashion for printing actions, and a sufficient time may not be given to allow the piezoelectric element P to be fully discharged. Moreover, the charging and discharging intervals may vary from time to time. In this situation, the piezoelectric element P may or may not have a residual energy when the charging of the element P is started. The amount of the residual energy may fluctuate. Accordingly, the maximum amount of displacement of the piezoelectric element or the operating position (i.e., fully displaced position) of the element varies from one activation cycle to another, whereby the drive force produced by the piezoelectric element P varies from time to time. To avoid this problem, the driver circuit discussed above must be adapted to provide a comparatively long time for discharging the piezoelectric element so that no residual energy is left prior to the charging in each activation cycle.
A dot-matrix impact printer utilizing a piezoelectric element as described above is generally constructed so as to include (a) a print head having a piezoelectric actuator to and from which an electric energy is applied and removed, and a print wire operated by the piezoelectric actuator, to form dots on a recording medium, (b) a feeding device for effecting a relative movement between the print head and the recording medium in a printing direction, (c) an actuator control device responsive to printing data, for controlling the piezoelectric actuator to move the print wire between a printing position in which the print wire is pressed against the recording medium to form a dot at one of predetermined printing positions which are spaced from each other at a predetermined pitch in the printing direction, and a non-printing or retracted position in which the print wire is spaced away from the recording medium. The print wire is moved to the printing position by one of the application and removal of an electric energy to and from the actuator, and is returned to the non-printing position by the other of the application and removal of the electric energy.
Generally, the piezoelectric type print head using a piezoelectric element or elements as an actuator incorporates a mechanism for amplifying an amount of displacement of the piezoelectric element to be transmitted to the print wire, so that the amount cf movement of the print wire between the printing and non-printing positions is larger than the amount of displacement of the piezoelectric element. The non-printing position of the print wire is elastically defined or established by the displacement amplifying mechanism. For accurate establishment of the non-printing position of the print wire, the displacement amplifying means may be provided with a stop disposed adjacent to the print wire or a member which moves with the print wire, so that the non-operating or fully retracted position of the print wire is determined by abutting contact of the print wire with the stop. In the printer having this type of print head, the piezoelectric element is energized and deenergized in response to a drive command for activating the print head. Upon energization of the piezoelectric element by application of a voltage thereto, the piezoelectric element is displaced in a forward direction, to cause an advancing movement of the print wire from the non-printing position to the printing position, whereby the operating end of the print: wire is pressed against a recording medium. When the piezoelectric element is deenergized by removal of the voltage therefrom, on the other hand, the piezoelectric element is displaced in a reverse direction, to retract the print wire away from the recording medium back to the non-printing position. It is possible that the print wire is biased toward the printing position by suitable biasing means such as a resilient member so that the print wire is retracted by applying a voltage to the piezoelectric element, and is advanced to the printing position under the biasing force of the biasing means by removing the voltage from the piezoelectric element.
The piezoelectric element may be replaced by other actuators such as a solenoid coil. A print head using a solenoid coil as an actuator for operating a print wire generally includes (a) an armature which is displaced according to a magnetic force produced by the solenoid coil and which is returned to an original position thereof upon deenergization of the solenoid coil, (b) a displacement transmitting mechanism for transmitting a displacement of the armature to the print wire, (c) a stop disposed adjacent to the print wire or a member which moves with the print wire, for defining the non-printing position of the print wire. In a printer having this type of print head, the solenoid coil is energized and deenergized in response to the drive command for activating the print wire. When the solenoid coil is energized, the armature is displaced in a forward direction, to advance the print wire from the non-printing position for forcing the prin.t wire against the recording medium. When the coil is deenergized, the armature is returned in a reverse direction to retract the print wire away from the recording medium back to the non-printing position. Alternatively, the armature is biased in the forward direction by suitable biasing means, and the solenoid coil is energized to produce a magnetic force to attract the armature in the reverse direction against the biasing force of the biasing means and hold the armature at the original retracted position. Upon generation of the drive command, the solenoid coil is deenergized to allow the armature to be moved in the forward direction under the biasing force of the biasing means, thereby advancing the print wire to the printing position.
Since it is desired to effect a dot-matrix printing operation at a speed as high as possible, it is proposed to control a head driver circuit of the dot-matrix printer such that the application and removal of an electric energy to and from the print wire actuator such as the piezoelectric element or solenoid coil as discussed above are completed in time periods as short as possible. That is, it is proposed to reduce not only the wire advancing time necessary to advance the print wire from the non-printing position to the printing position for pressing contact with the recording medium, but also the wire retracting time necessary to retract the print wire from the printing position back to the non-printing position.
However, there is a limitation in shortening the wire retracting time. If the wire retracting time is too short, the speed of movement of the print wire approaching the non-printing position is so high that the force of abutting contact of the print wire with a stop at the non-printing position becomes excessively large, where the non-printing position is established by the stop. If the non-printing position is established without using a stop, the print wire overruns past the non-printing position. In either case, the print wire tends to rebound in the advancing direction, and may sometime reaches the printing position. For this reason, the discharging or deenergizing time of the piezoelectric element, solenoid coil or other actuator for the print wire should not exceed a certain limit, so as to prevent the print wire from undergoing an excessively large rebounding motion sufficient to effect printing of a dot, when the print wire is returned to the non-printing position upon discharging or deenergization of the print wire actuator. This is one of the factors that hinder an improvement in the printing speed of the dot-matrix printer.